Fluxless bonding of silicon wafers to molybdenum substrates using electroplated tin-rich solder

Abstract

A novel fluxless bonding process of silicon wafer on molybdenum substrate is successfully developed. Si-to-Mo bonding can be used for packaging power devices, especially when a device consists of an entire wafer. 300 A Cr layer and 1,000 A Au layer are first deposited on Si wafers and Mo substrates. The Cr/Au dual layer is used as underbump metallurgy and seed layer of electroplating. To reduce plastic shear strain on the solder in a bonded pair, thick Sn layer (70 μm) is electroplated over Mo substrates having Cr/Au structure, followed immediately by thin (0.1 μm) Ag layer. This Ag layer acts as the capping layer to prevent inner Sn from oxidation. The bonding process is performed in 50 millitorrs vacuum to inhibit oxidation. The bonding condition is 290 °C for 15 min without the use of any flux. The bonding layer thickness is controlled at 50 μm by small spacers placed between Si wafer and Mo substrate. Microstructure and composition of the joints are studied under scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Scanning acoustic microscopy (SAM) is also used to verify the quality of joints over the entire sample. Without using any flux, high quality and uniform bonding layer is achieved. The composition of the joint is more than 97 at.% Sn. No intermetallic compound layers exist in the joint. This novel fluxless bonding process should be valuable in packaging large high power devices.